The present invention relates to a power line monitoring device or system, and more particularly to an improved system of this nature for monitoring line isolation and the existence of fault currents resulting from fault loads on the power line or lines.
In various installations wherein a load or loads are connected to a power supply, there is the possibility of a fault condition occurring wherein there is a current flow to ground, as in the case where a low leakage impedance occurs between one or both power lines to ground because of faulty insulation or other causes. In many instances a hazardous leakage current condition results. It thus is desirable to sense or monitor the existence of faults to ground, particularly those occurring on an isolated ac power distribution network. A major application for monitoring devices of this nature is for hospital use because of the rapidly increasing use of electro-medical apparatus in the care of patients. Such apparatus are used in hospital intensive care wards, surgeries, and other hospital locations, and the monitoring of current leakage to ground is paramount for the safety of patients.
Various types of monitoring devices for monitoring such leakage current to ground have been devised. Exemplary systems are described in U.S. Pat. No. 2,999,231, No. 3,066,284 and No. 3,666,993. The typical system generally involves the use of several test impedances connected to the power lines for determining the existence of predetermined fault currents.
The present invention contemplates, in a preferred embodiment thereof, a line isolation monitor system which is relatively simple, sensitive and efficient, and is particularly adaptable to hospital and other uses. In a hospital application, for example, the conventional power supply includes a balanced isolated transformer with a floating secondary. When no fault condition exists, the patient can be connected to either side of the secondary without danger. A high quality ground usually is used throughout the system wherein beds, window frames, electrical instruments, and the like are all connected to a common ground point, and with the floating power system, even if the patient touches one side of the secondary there is no problem. However, in the event of the occurrence of a leakage path involving current of sufficient magnitude, a hazardous condition exists.